Inventions described in Japanese Patent No. 2,561,023, Japanese Patent No. 2,605,827, and JP-A-4-291167 relate to radio-frequency detection techniques based on diode wave detection. FIGS. 36 and 37 are circuit diagrams exemplifying radio-frequency detection circuits in the prior art. In order to convert radio frequency power into a direct current, a half-wave rectification operation achieved by a diode D in FIG. 36 or FIG. 37 is utilized by way of example. On this occasion, in order to limit a DC bias to, for example, about 1 μA in consideration of low power consumption, a resistor in the order of several MΩ is required as a resistor for use in the detection circuit, in either of the cases of FIGS. 36 and 37 when a supply voltage of 3 V is employed.
FIG. 38 is a circuit diagram exemplifying the arrangement of a general or typical start signal output circuit in the prior art, and the circuit arrangement is such that the wave-detector circuit in FIG. 37 and a general differential amplifier are combined. Regarding such a prior art device, applications are also found in, for example, Japanese Patent No. 3,202,624 and JP-A-10-56333. In the circuit arrangement in FIG. 38, a capacitor C00 in FIG. 37 is arranged in series on a signal transfer line within a matching circuit (MC).
Meanwhile, as a prior art determination circuit which determines the level of an input potential, there has been generally and widely known, for example, a circuit which is described in “Guide to Electronic circuit Work for Learning by Fabrication” (authored by Seiichi Inoue and published by Sougou-Denshi Shuppan). Shown in FIGS. 39A and 39B is a prior art determination circuit (a wave-detector and detection circuit). Further, FIG. 39C is a waveform diagram showing the situation of the change of the output of the detection circuit shown in the circuit diagram of FIG. 39B. This circuit diagram shows the detection circuit of an ultrasonic distance measurement apparatus. As shown in FIG. 39B by way of example, a determination process in the prior art is executed by a comparator which is configured using an operational amplifier that requires a current of mA order.
Under the premise of a battery operation or the like, however, lowering power consumption as desired is difficult when the operational amplifier requiring the current of mA order is adopted. Further, when well-known heterodyne wave detection, for example, is performed, radio wave power at a low level of about −60 dBm can be detected. In such an apparatus, however, a signal generator, a LNA, a mixer, etc. must be always operated, and power consumption during a standby time period is therefore difficult to be suppressed. Thus, the target lower power consumption is not attained, either. On the other hand, the prior art sensing scheme based on the diode is difficult to enhance a sensing sensitivity. These circumstances will be major causes for making difficult the compatibility between the higher sensitivity and lower power consumption of the wave-detector circuit.
Moreover, in order to manufacture and put into practical use a start signal output circuit of wider applications, the following two problems will need be solved in addition to the above various problems:
(1) Problem of Adaptability to Temperature Environment
In a case where the start signal output circuit is to be utilized for, for example, an ETC or a “smart plate,” it should desirably be applicable within a temperature range of about −30° C. to +60° C.
(a) Immunity Against Lowering of Supply Voltage
FIG. 40 is a graph exemplifying the discharge-temperature characteristics of a lithium battery which is commercially available. The battery is a cylindrical manganese-dioxide lithium battery. Its nominal voltage is 3 V, supposed continuous standard load is 20 mA, and service temperature range is −40° C. to +70° C. Shown in the graph are results obtained when, with a discharge load set at 60ω, output potentials were measured over about 30 hours. As seen from FIG. 40, the output voltage of the dry cell depends greatly upon the temperature, and it becomes drastically lower than the initial voltage of the battery at normal temperatures, in some situations of use thereof.
It is accordingly understood that, in a case where the battery operation is intended, especially in a case where the use of the battery in a cold district, for example, is also supposed for the desired start signal output circuit, an intense immunity against the lowering of the supply voltage is required.
Meanwhile, when a circuit operation is considered, the DC component of the sensing diode need be amplified in the start signal output circuit. Therefore, the output point P1 of the diode detection circuit and the input point P2 of the amplifier in the prior art example (FIG. 38) need be directly connected. As a result, the biases of the sensing diode and amplifier cannot be made independent. Accordingly, in order to hold a sensed output constant even when the supply voltage Vcc fed by the battery has lowered, the potential of the output point P1 of the diode detection circuit, the potential of the input point P2 on the amplifier side and the bias voltage Vbb for use in the amplifier need be lowered with the lowering of the supply voltage versus time, while they are always kept in balanced fashion.
In, for example, the start signal output circuit 900 shown in FIG. 38, even in a case where the resistances of resistors R1 and R2 are properly set so that the diode detection circuit and the amplifier may appropriately operate at a certain value of the supply voltage Vcc, a biased state changes when the supply voltage Vcc lowers.
On this occasion, however, unless the bias of the amplifier properly lowers in balanced fashion likewise to the bias of the diode detection circuit, the start signal output circuit 900 does not appropriately operate as a whole when the supply voltage Vcc lowers greatly. That is, the circuit of the prior art arrangement (example: start signal output circuit 900 in FIG. 38), which does not have a flexible corresponding mechanism (balancing function) capable of coping with such a problem relevant to the lowering of the output voltage of the power supply, is apprehended to erroneously operate in some cases.
Even if a precise operation is possible, the appropriate range of the supply voltage on that occasion narrows, and hence, a serviceable time period shortens in the case of using the battery as the power supply.
(b) Immunity Against Noise
Further, in a high-temperature environment, thermal noise, flicker noise, etc. are liable to develop within a circuit as is often observed in case of using, for example, MOSFETs for the circuit. Therefore, the start signal output circuit needs to have a predetermined immunity against the noises which develop within the circuit. That is, in order to realize the start signal output circuit whose sensitivity is high even in the environment of, for example, the wide range of temperatures as described above, a high S/N ratio need be secured in relation to the internal noises.
(2) Problem of Power Consumption of Determination Circuit
The determination circuit is required to determine ON/OFF (whether or not a signal has arrived) after the conversion from the RF into the DC. As also seen from the foregoing circuit arrangement in FIG. 39B, the addition of such a determination circuit leads to increase of power consumption. That is, it is not easy to operate the desired start signal output circuit as a whole, always at a low voltage and at a low current.
Considered as peripheral devices which require currents of mA order as can be supposed in the case of fabricating the RF/DC converter are, for example, a DC—DC converter or a regulator for ensuring a bias voltage, and an operational amplifier or a voltage comparator for amplifying and binarizing a result outputted from the RF/DC converter. However, when an apparatus configuration in which also the peripheral devices must be always operated is adopted, the start signal output circuit of very low power consumption cannot be eventually constructed as the whole apparatus.
Moreover, such a problem becomes an issue or is actualized especially in cases of battery drive, etc. In order to suppress the power consumption of the whole circuit to a low level, accordingly, the start signal output circuit will need to include a special determination circuit of low power consumption by itself.
Further, the problem of a high resistance need be taken into consideration. When the high resistance of about several MΩ is formed on an IC chip as shown in FIG. 37 by way of example, the resistance element becomes long, and hence, a large space is necessitated. As a result, the area of the resistance element facing the ground enlarges. Thus, a parasitic capacitance and a parasitic resistance appear between the resistance on the chip and the ground within a substrate, and radio frequency power leaks to the substrate. With such a simple configuration, therefore, radio frequency power at, for example, a frequency of 5.8 GHz and a low level of −60 dBm cannot be converted into a direct current by the diode.
Further, as other problems concerning the diode wave detection, it is pointed out that, in a case where the load resistance becomes high, the applied voltage between the anode and cathode of the diode becomes small, so radio frequency power cannot be converted into a direct current, and that, even in a case where the radio frequency power is successfully converted into the direct current, it is indistinguishable whether the output potential fluctuation of the diode is ascribable to the fluctuation of a bias or the DC conversion of the RF power.
The present invention is made in order to solve the above problems, and has an object to realize a start signal output circuit which is small in size, high in sensitivity and low in power consumption.
A further object of the invention is to realize a start signal output circuit whose serviceable temperature range is wide even in case of battery drive.
However, each object mentioned above may be individually accomplished by any and at least one of inventions to be described later, and the individual inventions of the present application shall not necessarily guarantee that means capable of simultaneously solving all the above problems be existent.